with any external programming language which is able to read input
(arguments)
from command-line and to print answer on stdout. If user uses some
function not defined in Kalkar, then Kalkar looks for a script with
same name from a
designated folder, runs it with arguments and reads output.

Something is already done -- simple expressions and defining variables
and lists and exernal functions. And there is an user interface for it.

Missing:

implementation of functional language. Grammar is ready.

configuration part -- what program to use for executing different
scripts.

better user interface -- main problem with current ui is that one
can
get previous expression back with down arrow and this is unnatural for
people used with terminals.